1. Field of the Invention
The present invention relates generally to measurement systems, and more particularly, to methods and systems for measuring aberrations in a wave front.
2. Related Art
When an otherwise planar wave front of an optical signal, such as, for example, a laser beam, travels through a medium with a variable index of refraction, its wave front may become distorted. When the transmission distance through the medium is relatively small and the aberrations are caused by a flow with a changing density field, these optically aberrating effects are referred to as aero-optical effects. These optically aberrating effects often degrade the performance of optical systems using such optical signals. These optical systems may include, for example, weaponry related systems and optical free-space communications systems.
Aberrations of an optical wave front may be caused by the wave front passing through one or more variable-index turbulent flow fields. Such flow fields may originate in a mixing layer between two dissimilar-index flow streams (commonly referred to as “two-index mixing”). Or, aberrations of a wave front may result from the optical signal passing through separated flows, boundary layers, and free shear layers. The latter scenarios often occur when using lasers on flight vehicles traveling at high flight speeds (e.g., Mach numbers greater than 0.3).
The optically-aberrating effects of high-speed, turbulent boundary layers have been the subject of research since the early 1950's, which produced the first theoretical formulation for the optically-aberrating effects based on statistical measures of the turbulence. Work on turbulent boundary layers and separated shear layers intensified in the late 1960's and through the 1980's due to an interest in placing lasers on aircraft.
A Malley-Derivative Sensor, hereafter referred to as a Malley probe is an optical instrument that can make direct, accurate measurements of dynamically-distorting wave fronts, including the characteristics of the Optical Path Difference (OPD) and Optical Path Length (OPL) of a wave front. A single-beam instrument for measuring OPDrms, a statistical measure of OPD, was described in an article by Malley, M., Sutton, G. W., and Kincheloe, N., “Beam Jitter Measurements for Turbulent Aero-Optical Path Differences,” 31 Applied Optics at 4440-4443 (1992), which is incorporated by reference in its entirety herein; however, the Malley, Sutton and Kincheloe instrument required a priori information about the convective speed of the aberrating flow and provided only a statistical measure of the aberrating character of the flow.
The Malley, Sutton and Kincheloe instrument further cannot be used to determine the convection velocity of the aberrations, and the analysis of the instrument's single beam could not provide realizations of the aberrated wave fronts.